Space charge compensation



July 22, 1941. w. F. FATHAUER SPACE CHARGE COMPENSATION Filed June 6,1940 INVENTQR. v WaZZe/F Fal /2621166 7 11.. adapted for excitation forthe emission of electrons which, impinging upon the anode, result inPatented July 22, 1941 SPACE CHARGE COMPENSATION Walter F. Fathauer,Highland Park, 111., assignor to- General Electric X-Ray Corporation,Chicago, 111., a corporation of New York Application June 6, 1940,Serial No. 339,109.

2 Glaims.

My invention relates general to electronics and has more particularreference to the control of X-ray'tubes;

An important object of the present invention is to maintain therelationship of voltage to current as a constant function duringoperation of devices of the character mentioned, operation of the tubein accordance with the present invention being accomplished at all timesin accordance with a pred'etermned voltage current relationship whichmay be graphical-1y illustrated as a single curve;

Another important object is to provide means to compensate for spacecharge efiects in the operation of an electronic device, comprisingananode and a cathode adapted to be excited for electron emission, sothat the relationship of voltage across the device to anode currentcomprises a constant function regardless of the extent cathodeexcitation.

Among the advantages obtained by causing operation of the device, inaccordance with an invariable voltage current relationship, is that theperformance of the device in terms of anode current may be accuratelypredetermined or predicted for all operating voltages, therebyminimizing. uncertainty and making for precise results, which are ofimportance especially where X-ray technique is involved;

The foregoing and nun'ierous other important objects, advantages andinherent functions of the invention will become apparent as it is morefully understood from the following description, which, taken inconnection with the accompanying drawing, discloses a preferredembodiment of the invention;

eration of an electronic device embodying an anode and a cathode adaptedfor excitation for the emission of electrons; and

Figure 21 isa diagram of connections for the control of an electron flowdevice, in accordance with the teachings of my present invention.

To illustrate the invention, I have shown in the drawing an X-ray tubell comprising a sealed envelope i3 containing a cathode I5 and an anodeThe cathode I5 is shown as a filament the production of X-rays. Theelectrons emitted 'by the cathode are attracted toward and impinge. uponthe anode only so long as it remains electrically positive with respectto the cathode.

If and when the anode becomes electrically negative with respect to thecathode, it becomes electron repellent.

Upon excitation of the cathode L5, electrons will be emitted: therebyand will flow toward and impinge upon the plate I? and form a. currentconducting path between anode and cathode, along which flows the tubecurrent under the influence of electricalpotential applied between anodeand cathode. The electrons emitted by the cathode, unless driven towardthe anode under the influence of potential of appreciable magnitude,congregate in-the space near the cathode and collectively constitute acloud of electrons, which is-known as-the space charge. These electrons,being negatively charged, tend to repel other electrons leaving thecathode, and a state of equilibrium will be established between thepressure of oncoming electrons and the pressure exerted by the electronswhich have already escaped from the cathode. A- number of electronswill, of course, reach the anode and give up their charge, therebycreating X-rays. When the voltage between anode and cathode isexceedingly low, many of the-electrons donot reach the anode even thoughemitted by the cathode, because the velocity with which they are emittedby the cathode is insufficient to carry them to the anode through thespace charge of mutually repellent electrons surrounding the plate.Under excessively low voltage conditions, increasing the number ofelectrons emitted by the cathode will have very little eiiect' on theanode current, and to increase anode current, it is necessary toincrease the voltage between anode and cathode. The effect of the spacechargeis tohamper the movement of electrons toward the anode from thecathode and thus limit anode current, the space charge effect beingparticularly noticeable under low voltage conditions, and being shown atthe lower end of the graph in Figure 1 of the drawing. As voltage acrossanode and cathode is increased, current increases relatively slowlyuntil the space charge effect isovercome, current thereafter increasingmore rapidly in accordance with a straight line function.

The current flow through the tube also is limited by the availablesupply of electrons emitted by the cathode and by the behavior of theelectrons after they leave the cathode. The available supply ofelectrons is determined by the intensity of cathode excitation,proportionally more electrons being emitted as cathode excitation isincreased. For every degree of excitatiom'there is a limit to the numberof electrons that may escape from the cathode, so that maximum possibleanode current is determined by the degree of excitation, ordinarilyexpressed in terms of filament current. To attract all of the emittedelectrons to and cause the same to impinge on the anode requires adefinite optimum potential difference between anode and cathode, therebeing a difierent optimum for every degree of cathode excitationthroughout the operating range of the tube. When all of the electronsemitted by the cathode impinge upon the anode, the tube is in what maybe designated as a condition of saturation. When a tube is in saturatedcondition, current flow between anode and cathode is limited to acertain maximum value, even though voltage across the tube may beincreased, current limitation being caused by the fact that aftersaturation, only as much current as can be carried by the emittedelectrons may flow between cathode and anode.

As tube voltage is increased toward a condition of saturation, thevoltage-current relationship is affected by an effect similar to thespace charge effect previously mentioned, and the current increase, inresponse to voltage increase, gradually is reduced as the saturatedcondition is reached, as illustrated at the upper end of the graph,Figure 1 of the drawing, in Which the curves H and L represent thecurrent-voltage curves for the same tube, operating respectively withhigh and low cathode excitation. It will be apparent that the tube maybe adjusted for operation at any point between the limits defined by thecurves H and L, depending upon the adjustment of applied voltage andcathode excitation, anode current thus being determined by theadjustment of a plurality of variables. In practical X-ray technique,the necessity of carefully adjusting both applied voltage and filamentexcitation consumes valuable time, and the fact that anode current ofany value between points M and N may be attained at the same appliedvoltage, represented by the line V, makes for confusion. I propose tosimplify X-ray technique by providing voltage current control through asingle, manually operable handle so that for each position of thecontrol handle, there will be a corresponding flow of anode current,which will always be the same, thus avoiding confusion and the chance ofobtaining excessive current flow through improper control of filamentexcitation. I propose to accomplish the control by decreasing filamentexcitation as the tube voltage is increased, and vice versa, so that, atlow voltages, the space charge effect is to some extent overcome bymaximum filament excitation, while current is limited at high voltagesby saturation, as indicated by the curve C in Figure 1, in which onlyone value of anode current, represented by the point P, is obtained inresponse to tube voltage, represented by the line V.

As shown in Figure 2 of the drawing, power for actuating the device H issupplied from a suitable source shown as a power line l9, preferablysupplying alternating current power. The power is delivered from theline l9, preferably through a control switch 2| and through anadjustable auto-transformer 23, t the primary winding of a powertransformer 21, the secondary winding 23 of which is interconnectedbetween the anode and cathode of the tube for the purpose of supplyingelectrical power for the operation of the tube. The power source is alsoconnected through the auto-transformer and an adjustable current controldevice 3| with the primary winding 33 of a current limiting transformer35, the secondary winding 31 of which is connected in series with theprimary winding 39 of a filament transformer 4|, the secondary winding43 of which is connected in series with the filament I5 of the X-raytube. The primary winding of the filament transformer is powered fromthe source [9 through the auto-transformer 23 and an adjustable resistor45, so that the equipment may be adjusted by means of the resistor 45for operation in accordance with predetermined conditions, theadjustable device 3| serving to alter the value of cathode excitationduring the operation of the device.

One side of the line H3 is connected through the switch 2| with one sideof the primary winding 25 of the transformer 21 by means of a conductor41, to which conductor one end of the auto-transformer coil 23 iselectrically connected. The other side of the line is connected throughthe switch 2| with an adjustable contact member 49 co-operativelyassociated with the autotransformer coil 23. It is, of course, notessential to utilize an adjustable connection at 49, but the lineconnection may be made permanently on the winding 23, as at the endthereof. It is desirable, however, to provide an adjustable connectionin order to set the apparatus to deliver a predetermined voltage acrossthe coil 23.

The coil 23 has associated with it an adjustable contact member 5|,which is electrically connected by means of the conductor 53 with theside of the primary winding 25 of the transformer. '2'| remote from theconductor 4'! so that. by adjusting the position of the adjustablecontact 5| on the auto-transformer coil 23, any desired potential withinthe operating limits of the apparatus, as determined by the setting ofthe member 49, may be applied on the primary winding of the transformer21 to thereby apply a desired operating voltage between the anode andcathode of the tube. The system also includes a cathode exciting circuitextending from one side of the line I9 through the switch 2| and thencethrough a conductor 51, the adjustable rheostat 55, the primary winding39 of the filament transformer, and the secondary winding 3? of thecurrent limiting transformer 35, and thence to the conductor 41 whichserves as a common return conductor in the system. The system,furthermore, includes a circuit for regulating current flow in thecathode exciting circuit, said regulating circuit extending from oneside of the line I9 through the switch 2|, and a conductor 59 connectedwith a slide contact 6| having adjustable relationship with the resistor3|, which is connected through the primary winding 33 of the limitingtransformer 35 to the common return conductor ll. By varying therelationship of the contact member 6| with respect to its associatedresistor 3|, the limiting transformer 35 may be caused to alter thecurrent flowing in the cathode exciting circuit. The cathode excitingcircuit includes the primary winding of the filament transformer,current flow through which may also be adjusted by means of the rheostat45. The slide members 5| and 6|, which respectively control the voltageapplied between the anode and cathode of the tube and the flow ofcathode exciting current, are interconnected, as by mounting the same ona common manually operable member 53, which, in the illustratedembodiment, is shown as a lever member on which both of the contactdevices 5| and 6| are mounted. By moving the member 63, both contactdevices and 6| may be caused simultaneously to be adjusted respectivelyon the autotransformer coil 23 and on the coil 3|, therebysimultaneously altering the voltage applied between anode and cathodeand the cathode exciting current delivered through the transformer 4|;and the parts are arranged so that the cathode exciting current isincreased as the tube voltage is decreased, and vice versa, therelationship of the cathode exciting current to tube voltage being aninverse function of any desired character, depending upon the selectedconstants of the circuits involved.

It will be seen from the foregoing description that I have providedmeans whereby to control the operation of an electronic device through asingle manual control element which serves to determine cathodeexcitation as a function of tube voltage, to the end that the device maybe operated on a constant tube voltage-anode current curve. The controlassures the production of a definite anode current and correspondingX-ray intensity, in response to any value of tube voltage applied,thereby eliminating confusion and simplifying the operation of theapparatus. At the same time, by providing for the increase of cathodeexcitation at low voltages, the space charge eifect is reduced, and byreducing cathode excitation at high voltages, excessive anode current isprevented.

It is thought that the invention and its numerous attendant advantageswill be fully understood from the foregoing description, and it isobvious that numerous changes may be made in the form, construction andarrangement of the several parts without departing from the spirit orscope of the invention, or sacrificing any of its attendant advantages,the form herein disclosed being a preferred embodiment for the purposeof illustrating the invention.

The invention is hereby claimed as follows:

1. The combination, with an X-ray tube having an anode and an excitablecathode, of a transformer having a secondary winding connected betweensaid anode and cathode and a primary winding connectible with a powersource through adjustable power supply means for varying the voltageapplied through said transformer between the anode and cathode, acurrent transformer having a secondary winding connected with saidcathode and a primary winding connectible in circuit with said powersource, said circuit including the secondary winding of a controltransformer, the primary winding of which is connectible, in associationwith an adjustable current flow control device, with said power source,and means drivingly interconnecting the adjustable portions of saidcurrent flow control device and said power supply means.

2. The combination, with an X-ray tube having an anode and an excitablecathode, of a transformer having a secondary winding connected betweensaid anode and cathode and a primary winding connectible with a powersource through adjustable power supply means for varying the voltageapplied through said transformer between the anode and cathode, acurrent transformer having a secondary winding connected with saidcathode and a primary winding connectible in circuit with said powersource, an adjustable ci1' cuit control device in said circuit, saidcircuit including the secondary winding of a control transformer, theprimary winding of which is connectible, in association with anadjustable current flow control device, with said power source, andmeans drivingly interconnecting the adjustable portions of said currentflow control device and said power supply means.

WALTER F. FATHAUER.

